The hydrogenation of the unsaturated fatty acid constituents of an edible oil's triglycerides is carried out to produce a more oxidatively stable product and/or change a normally liquid oil into a semi-solid or solid fat with melting characteristics designed for a particular application. Most commercial oil hydrogenation plants use Raney or supported nickel catalyst, where the chemical catalytic reaction is carried out at a high temperature (typically 150-225 C.) and a hydrogen gas pressure in the range of 10-60 psig. These conditions are required to solubilize sufficiently high concentrations of hydrogen gas in the oil/catalyst reaction medium so that the hydrogenation reaction can proceed at acceptably high rates. The hydrogenation rate and fatty acid product distribution has been shown to be dependent mainly on temperature, pressure, agitation rate, and catalyst type and loading. Unfortunately, high reaction temperatures promote a number of deleterious side-reactions including the unfavorable production of trans isomers and the formation of cyclic aromatic fatty acids.
An alternative method to edible and nonedible oil and fatty acid hydrogenation by a traditional chemical catalytic reaction scheme is a low temperature electrocatalytic (electrochemical) route, where an electrically conducting catalyst (e.g., Raney nickel or platinum black) is used as the cathode in an electrochemical reactor. Atomic hydrogen can be generated on the catalyst surface by the electrochemical reduction of protons from the adjacent electrolytic solution. The electro-generated hydrogen then reacts chemically with unsaturated fatty acids in solution or in the oil's triglycerides. The overall oil hydrogenation reaction sequence is as follows: EQU 2H.sup.+ +2e.sup.-.fwdarw.2H.sub.ads (1) EQU 2H.sub.ads +R--CH.dbd.CH--R.fwdarw.R--CH.sub.2 --CH.sub.2 --R (2)
where R--CH.dbd.CH--R denotes an unsaturated fatty acid. An unwanted side reaction that consumes electro-generated H.sub.ads (i.e., current) but does not effect the organic product yield is the formation of H.sub.2 gas by the combination of two adsorbed hydrogen atoms, EQU 2H.sub.ads.fwdarw.H.sub.2 (gas) (3)
All electrochemical reactors must contain two electrodes, a cathode for reduction reactions such as that given by Equation 1 and an anode at which one or more oxidation reactions occur. For a water-based electrolytic solution, the anode reaction is often the oxidation of H.sub.2 O to O.sub.2 gas, EQU H.sub.2 O.fwdarw.1/2O.sub.2 +2H.sup.+ 2e.sup.- (4)
In organic electrochemical syntheses where two or more reactions occur at the same electrode, the effectiveness of the primary electrode reaction is often gauged by the reaction current efficiency. During the electrochemical hydrogenation of edible or non-edible oils, this quantity is a measure of the amount of electro-generated hydrogen which combines with an oil's unsaturated fatty acids (according to Equation 2), as opposed to the amount of atomic hydrogen lost as H.sub.2 gas (Equation 3). The current efficiency is computed from the change in total moles of double bonds in the oil or fatty acid (as determined from the gas chromatography fatty acid profiles of initial and final samples of the reaction medium) and the total charge passed in an electrolysis, as noted by the product of the current density (A/cm.sup.2), the geometric electrode area (cm.sup.2), and the time of current passage (seconds), EQU Current Efficiency(%)=(_moles of double bonds)(2 equiv/mole)F/C (5)
where F is Faraday's constant (96,487 C/equiv.) and C is the total coulombs passed in electrolysis (the total coulombs is given by the arithmetic product of the current density, geometric electrode area, and time). For the cathodic reaction system where electro-generated H either adds to the oil or two hydrogen atoms combine to form H.sub.2, a current efficiency below 100% provides a direct measure of the fraction of current consumed by the H.sub.2 gas evolution reaction (cf. Equation 3).
The hydrogenation of the fatty acid constituents of an edible oil's triglycerides is a particularly attractive reaction to examine in an electrocatalytic scheme for the following reasons: (1) low reactor operating temperatures minimize unwanted side reactions and the deleterious thermal degradation of the oil, (2) normally, only 25%-50% of the double bonds in an oil are hydrogenated, thus, eliminating the common problem in electrochemical reactors of low hydrogenation current efficiencies when the unsaturated starting material is nearly depleted, (3) the high molecular weight of the starting oil (892 g/mole for refined soybean oil) means that the electrical energy consumption per pound of hydrogenated product will be low even though the saturation of a double bond requires 2 F/mole of electron charge, and (4) when water is used as the anode reactant and source of H (according to Equation 4), the electrochemical oil hydrogenation method circumvents the need to produce, store, compress, and transport H.sub.2 gas.
Since hydrogen is generated in-situ directly on the catalyst surface in an electrocatalytic reaction scheme, high operating temperatures and pressures are not required. By maintaining a low reaction temperature, it may be possible to minimize unwanted isomerization reactions, thermal degradation of the oil, and other deleterious reactions. By passing a high current through the catalyst (i.e., by maintaining a high concentration of atomic hydrogen on the catalyst surface), the hydrogenation rate of the oil may be kept high, even at atmospheric pressure and a low or moderate reaction temperature.
Numerous studies have shown that low hydrogen overpotential electrically conducting catalysts (e.g., Raney nickel, platinum and palladium on carbon powder, and Devarda copper) can be used to electrocatalytically hydrogenate a variety of organic compounds including benzene and multi-ring aromatic compounds, phenol, ketones, nitro-compounds, dinitriles, and glucose [see, for example, T. Chiba, M. Okimoto, H. Nagai, and Y. Takata, Bulletin of the Chemical Society of Japan, 56, 719, 1983; L. L. Miller and L. Christensen, Journal of Organic Chemistry, 43, 2059, 1978; P. N. Pintauro and J. Bontha, Journal of Applied Electrochemistry, 21, 799, 1991; and K Park, P. N. Pintauro, M. M. Baizer, and K. Nobe, Journal of the Electrochemical Society, 16, 941, 1986]. These reactions were carried out in both batch and semi-continuous flow reactors containing a liquid-phase electrolytic solution. In most cases the reaction products were similar to those obtained from a traditional chemical catalytic scheme at elevated temperatures and pressures.
Pintauro [U.S. Pat. No., 5,225,581 Jul. 6, 1993] and Yusem and Pintauro [Journal of the American Oil Chemists Society, 69, 399, 1992] showed that soybean oil can be hydrogenated electrocatalytically at a moderate temperature, without an external supply of pressurized H.sub.2 gas. Experiments were carried out at 70 C. using an undivided flow-through electrochemical reactor operating in a batch recycle mode. The reaction medium was a two-phase substance of soybean oil in a water/t-butanol solvent containing tetraethylammonium p-toluenesulfonate (hereafter denoted as TEATS) as the supporting electrolyte. In the experiments the reaction was allowed to continue for sufficient time in order to synthesize a commercial-grade "brush" hydrogenation product (25% theoretical conversion of double bonds). Hydrogenation current efficiencies in the range of 50-80% were obtained for apparent current densities of 0.010-0.020 A/cm.sup.2 with an oil concentration between 20 and 40 wt/vol % in the water/t-butanol/TEATS electrolyte. The electro-hydrogenated oil was characterized by a somewhat higher stearic acid content, as compared to that produced in a traditional hydrogenation process. The total trans isomer content of the electrochemically saturated oil product, typically in the range of 8%-12% was lower than the 20%-30% trans product from a high-temperature chemical catalytic brush hydrogenation process.
In a second paper by Yusem, Pintauro, and co-workers [Journal of Applied Electrochemistry, 26, 989, 1996], soybean oil was hydrogenated electrocatalytically on Raney nickel powder catalyst at atmospheric pressure and moderate temperatures in an undivided packed bed radial-flow-through reactor, where Raney nickel catalyst powder was contained in the annular space between two concentric porous ceramic tubes and the flow of the reaction medium (a substance of oil in a water/t-butanol/tetraethylammonium p-toluenesulfonate electrolyte) was either in the inward or outward radial direction. For the brush hydrogenation of soybean oil, current efficiencies of 90-100% were achieved when T=75 C., the apparent current density was 0.010 or 0.015 A/cm.sup.2, and the reaction medium consisted of a substance of 10 or 25 wt/vol % soybean oil in water/t-butanol solvent with TEATS salt as the supporting electrolyte.
A serious drawback of the electrochemical oil hydrogenation work of Yusem, Pintauro and co-workers described above was the need to employ a mixed water/t-butanol solvent with a supporting electrolyte salt in order to stabilize the emulsified oil reaction medium and achieve a reasonably high ionic conductivity of the reaction medium. In the absence of a supporting electrolyte, the high resistivity of the reaction medium would cause no current to flow through the oil hydrogenation reactor. Since most salts are sparingly soluble in oils and unsaturated fatty acids, a two-phase reaction medium had to be employed where the salt was dissolved in either water or a mixture of water and t-butanol and the oil was dispersed as small droplets in the aqueous (or water/alcohol) mixture. Additionally, reasonable oil hydrogenation rates (i.e., reasonably high hydrogenation current efficiencies) could only be achieved using a quaternary ammonium salt supporting electrolyte (e.g., tetraethylammonium p-toluenesulfonate). Unfortunately, both the t-butanol co-solvent and the TEATS salt are not food-grade materials. Their use in a commercial edible oil or food-grade fatty acid hydrogenation process would require either proof that these compounds were not hazardous to human health or proof that the compounds can be completely removed from the oil product. Yusem showed, however, that small amounts of TEATS salt were present in the oil after electro-hydrogenation and product oil purification [G. Yusem, Ph.D. Dissertation, Tulane University, Dec. 20, 1994], was unable to be achieved). In order to correct the problems associated with this prior work on the electrochemical (electrocatalytic) hydrogenation of oils, a new divided electrochemical reactor configuration has been employed for oil/fatty acid hydrogenation where a polymeric cation-exchange membrane carries out the function of the solvent/supporting electrolyte. This so-called Solid Polymer Electrolyte (SPE) reactor is the subject matter of this patent. The use of such reactors for organic electrochemical oxidation and reduction reactions is not new. To date, however, no one has utilized such a reactor for the electrochemical (electrocatalytic) hydrogenation of edible/non-edible oils and fatty acids.
A solid polymer electrolyte reactor for organic species hydrogenation consists of separate anolyte and catholyte chambers separated by a thin wetted (i.e., hydrated or solvated) cation-exchange membrane. Porous (permeable) electrodes (one anode and one cathode) are attached to each face of the membrane, forming a "Membrane-Electrode-Assembly" (MEA), similar to that employed in solid polymer electrolyte hydrogen/oxygen fuel cells. Water can be circulated past the back-side of the anode, in which case water molecules are oxidized to O.sub.2 gas and protons, according to Equation 4. Alternatively, H.sub.2 gas can be oxidized to two protons and two electrons at the anode, EQU H.sub.2 (gas).fwdarw.2H.sup.+ +2e- (6)
The electrode reactions take place at electro-catalytic layers at the interfaces between the membrane and the permeable anode and cathode. Protons from H.sub.2 or H.sub.2 O oxidation at the anode migrate through the ion-exchange membrane under the influence of the applied electric field to the cathode catalyst component of the MEA where the protons are reduced to atomic and molecular hydrogen (Equations 1 and 3). This electro-generated hydrogen can then react with unsaturated fatty acids in an edible oil, for example, where the oil flows past the back-side of the cathode and permeates through the porous cathode structure to the reaction zone at the cathode catalyst/membrane interface. Ion (proton) conductivity occurs through the wetted (hydrated) cation-exchange membrane so that pure oil and distilled water can be circulated in the cathode and anode chambers, respectively. The close proximity of the anode and cathode on a MEA (the electrode separation distance is given by the thickness of the ion-exchange membrane which is typically in the range of 100 m-200 m) and the high ion-exchange capacity of the cation-exchange membrane (i.e., the high concentration of negatively charged moieties immobilized in the polymeric membrane) insures facile H.sup.+ transport between the anode and cathode and a small anode-cathode voltage drop during reactor operation at a given current. In such a reactor there is no liquid electrolyte (an aqueous or mixed solvent containing a dissociated supporting electrolyte salt) between the anode and cathode. For the hydrogenation of an edible oil, the use of a solid polymer electrolyte ("SPE") reactor eliminates the presence of supporting electrolyte salts and non-water co-solvents that contaminate the hydro-oil product.
SPE reactors have been examined previously for organic electrochemical syntheses (both oxidation and reduction reactions). The first applications of the SPE process for electro-organic synthesis were published by Ogumi et al. in Japan [A. Ogumi, K. Nishio, and S Yoshizawa, Electrochimica Acta, 26, 1779, 1981] and then by Tallec et al. in France [J. Sarrazin and A. Tallec, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 137, 183, 1982] and Grinberg et al. in Russia [V. A Grinberg, V. N. Zhuravleva, Y. B. Vasil ev, and V. E. Kazarinov, Electrokhimiya, 19,1447, 1983]. There have since been many publications by these and other authors concerning this organic electrochemical technique [see, for example, Z. Ogumi, H. Yamashita, K. Nishio, Z. Takehara, and S. Yoshizawa, Electrochimica Acta, 28, 1687, 1983 and Z. Ogumi, M. Inaba, S. Ohashi, M. Uchida, and Z. Takehara, Electrochimica Acta, 33, 365,1988 ]. Ogumi and co-workers, for example, examined the electrocatalytic reduction (hydrogenation) of olefinic compounds in a SPE reactor [Z. Ogumi, K Nishio, and S. Yoshizawa, Electrochimica Acta, 26, 1779, 1981], where the cathode reactant was either cyclo-octene, -methyl styrene, diethyl maleate, ethyl crotonate, or n-butyl methacrylate dissolved in either ethanol, diethyl ether, or n-hexane. The Membrane-Electrode-Assemblies in this study were composed of Pt, Au, or Au--Pt layers that were deposited onto the surface of a Nafion membrane generically known as a perfluorosulfonic acid cation-exchange membrane) (Nafion is a registered trademark of E. I. DuPont de Nemours Inc.).
Initial soybean oil hydrogenation experiments in a SPE reactor proved unsuccessful due to unacceptably low oil hydrogenation current efficiencies and the degradation of the cathode catalyst component of the MEA during multiple (long-term) experiments [Luke Stevens, M. S. Thesis, Tulane University, Dec. 18, 1995]. The SPE reactor contained membrane-electrode-assemblies purchased from Giner Inc., Waltham, Mass. that were composed of Pt-black (for the cathode) and RuO.sub.2 (for the anode) fixed to a Nafion 117 membrane. The cathode was composed of 20 mg/cm.sup.2 Pt-black (the thesis incorrectly states that the Pt catalyst loading for the cathode was 4 mg/cm.sup.2) with 15 wt % Teflon binder (Teflon is generically known as polytetrafluoroethylene.) Teflon is a registered trademark of E. I. duPont de Nemours Inc. and a platinized tantalum screen current collector. The anode was RuO.sub.2 (20 mg/cm.sup.2) with 25% Teflon binder (Teflon is generically known as polytetrafluoroethylene.) Teflon is a registered trademark of E. I. duPont de Nemours Inc. and either a platinum screen or platinized titanium screen current collector. The reaction was carried out by circulating either pure oil or oil diluted with heptane past the back-side of the cathode and either a dilute aqueous sulfuric acid or phosphoric acid solution past the back side of the anode. Electro-hydrogenation of the unsaturated fatty acid constituents of the oil was observed in most experiments, with a current efficiency of between about 18% and about 26%, for applied constant current densities between 0.050 and 0.20 A/cm.sup.2 and for temperatures between 50C. and 90 C. The low oil hydrogenation current efficiencies declined further to between 8% and 12% after using the MEA in two or more (up to ten) repeated oil hydrogenation experiments. Usually, an electro-organic process with these low product current efficiencies would be useless commercially due to the large losses in electrical energy and the unacceptably large size of the reactor(s) needed to hydrogenate a given amount of reactant. The unacceptably poor current efficiency performance of the reactor has been attributed to: (1) poorly designed MEAs, where the Pt-black cathode was too thick (i.e., the 20 mg/cm.sup.2 loading was too high) for oil reactant access to and oil product escape from the catalyst/membrane interface reaction zone and/or (2) the Teflon binder used in the cathode, which did not have the correct hydrophobic/hydrophilic character to allow for oil, water, protons and electro-generated H to meet at the catalyst/membrane interface reaction zone (i.e., if the catalyst binder is too hydrophilic, water will flood the reaction zone and there will be no access of oil to catalyst regions where H generation is occurring; similarly if the catalyst binder is too hydrophobic, oil will flood the catalyst and H generation will occur only on catalyst particles buried within the wetted cation-exchange membrane that are inaccessible to oil reactant). In addition to the low current efficiencies, these preliminary oil hydrogenation experiments suffered from a second drawback, that being the use of non-food-grade sulfuric and phosphoric acid in the water anolyte. Small amounts of these acids will be present with water in the cation-exchange membrane of the MEA and will contact the oil reactant.